Light emitting diode (LED) provides many advantages such as longer lifespan, less power consumption, higher illumination and more eco-friendly materials. With advance of LED fabrication process and lower cost thereof, LED is not only adopted on traffic lights or indication lights of electric appliances, it also can be used on environmental decoration or lighting fixtures. In order to allow the LED to adapt to the general light bulb, some techniques have been proposed in prior art. For instance, R.O.C. patent No. I293807 entitled “LED light bulb equipped with a constant current circuit” discloses an LED light bulb which includes a lamp cap, a lamp shell, a plurality of LEDs coupled in series and a step-down constant current circuit. The lamp cap has electrodes connected to a power source. The LEDs are connected to the step-down constant current circuit which provides a constant current to let the LEDs emit light. The LED light bulb can be directly mounted onto a conventional lamp holder when in use. However, since driving the LEDs requires a steady DC current, waste heat will be constantly generated and accumulated when the driving circuit converts AC power into DC power. Moreover, the DC power passing through the impedance of LEDs also generates a lot of waste heat. All these result in overhigh temperature after long-term use that could damage the LEDs or driving circuit, or shorten the lifespan thereof.
To remedy the aforesaid problem, many types of LED light bulbs equipped with heat dissipation structure have been developed. For instance, R.O.C. patent No. I338106 entitled “LED lamp set” discloses an LED lamp set which includes a light emission device containing an LED unit, a circuit board to drive and control the LED unit, and an internal lens. The circuit board is held in a second housing chamber of a metal lamp cup. The LED unit is held in a first housing chamber of the metal lamp cup and has a heat conductive baseboard and a high power LED chip mounted onto the baseboard. The drawings of this prior art show that the baseboard holding the high power LED chip is fastened to the metal lamp cup via a plurality of screws. The baseboard is held merely via these screws.
R.O.C. patent No. M350675 entitled “LED lamp set and shade structure of the same” discloses an LED lamp set that includes a latchable radiation fin set and an LED module. The latchable radiation fin set surrounds a housing chamber inside. The LED module is held in the housing chamber and connected to the radiation fin set. The drawings of this prior art show that the LED module has a circuit board fastened to the radiation fin set via screws.
Another R.O.C. patent No. M365440 also discloses a technique to fasten the LED baseboard via screws.
However, the aforesaid techniques of fastening the LED baseboard via the screws have many disadvantages. Please refer to FIGS. 1 and 2 for a conventional LED light bulb which includes a lamp shell 90, an LED baseboard 91, a heat sink 92, a power conversion board 93 and a power receiving base 94. The heat sink 92 has one end fastened to the power receiving base 94. The power conversion board 93 is interposed between the heat sink 92 and power receiving base 94. The heat sink 92 has a holding surface 921 at another end, a plurality of first screw holes 922 on the holding surface 921 and a wiring bore 923 running through the holding surface 921. The LED baseboard 91 holds a plurality of LEDs 911 and has a plurality of second screw holes 912 corresponding to the first screw holes 922. A plurality of screws 95 are provided to run through the screw holes 912 and 922 to fasten the LED baseboard 91 to the holding surface 921 of the heat sink 92. But such a structure also creates problems. For instance, if the LED baseboard 91 is fastened to the heat sink 92 via the screws 95, only the circumference of the screws 95 can be fully attached to the heat sink 92. Moreover, since the LED baseboard 91 is made of aluminum, it is easily to be deformed during heating of the LEDs 911 that are held on the LED baseboard 91 The drawings merely illustrate the deformation in a schematic manner. In practice, different deformed conditions occur due to different materials and temperatures. The deformation causes some portions spaced from the screws 95 cannot tightly contact with the heat sink 92 due to heat expansion and cold shrinkage. As a result, heat conduction speed between the LED baseboard 91 and heat sink 92 drops drastically, and the temperature of the LED baseboard 91 rises faster that causes even more obvious deformed condition. Hence a vicious cycle of poor cooling takes place.